Smiling is critical as a means of developing a new social skill and is a key social signal (Campos, 2009). Two types of smiling can be distinguished in infants:
- Reflexive smile: A smile that does not occur in response to external stimuli and appears during the first month after birth, usually during sleep.
- Social smile: A smile that occurs in response to an external stimulus, typically a face in the case of the young infant. Social smiling occurs as early as 4 to 6 weeks of age in response to a caregiver’s voice (Campos, 2005).

Daniel Messinger (2008) described the developmental course of infant smiling:
- 2-6 months: infants’ social smiling increases considerably, both in self-initiated smiles and smiles in response to others’ smiles
- 6-12 months: smiles that couple the Duchenne marker (eye constriction) and mouth opening occur during highly enjoyable interactions and play with parents
- Second year: smiling continues to occur in positive circumstances with parents. Increase in smiling occurs when interacting with peers. Increased awareness of the social meaning of smiles. Anticipatory smiling occurs - communicating preexisting positive emotion by smiling at an object and then turning their smile toward an adult.

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Sources:
Life-Span Development, John W. Santrock (Click for eBook)